1-axis and 2-axis solar trackers

ABSTRACT

A one-axis sun position tracking device with its rotation axis parallel to the rotation axis of the Earth, rotates perpetually at a constant speed in the opposite direction of the Earth&#39;s rotation. This device comprises a shaft that is aligned to the Earth&#39;s polar axis, one or more crossbars are rigidly attached to and perpendicular to the shaft, solar energy collectors are mounted on the crossbar and could rotate around the crossbar that defines declination angle. A self-latched declination angle adjustment mechanism keeps the declination angle constant at most of time. A drive mechanism keeps this solar tracker to rotate perpetually. An automatic and abrupt declination angle change will keep the declination angle updated to correct value each day. A similarly configured two-axis tracker that continuously updates its declination angle by a mechanism derived from a differential coaxial rotation. Two independent driving mechanisms control the speed and/or duration of the two coaxial rotations, and are programmed to eliminate all tracking errors from various sources.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of provisional application Ser. No.61/269,462, filed 2009 Jun. 26 by the present inventor.

FEDERALLY SPONSORED RESEARCH

Not applicable

SEQUENCE LISTING

None

BACKGROUND

1. Field

This invention relates to solar energy collections, specifically to sunposition tracking that is used in sunlight concentration and collection.

2. Prior Art

Sun position tracking is very important to solar energy collection,especially for solar concentrators. Different implementations have beeninvented, they could be categorized as 1-axis tracking and 2-axistracking. 1-axis tracking is simple, however, it is commonly believedthat 1-axis tracking has a poor tracking capability, that is not true ina special case; 2-axis tracking can have a very good tracking accuracy,however, it is usually very complicated because of the coupled rotationof 2-axis.

Most 2-axis tracking methods are using local coordinate system, orcalled horizontal mount. It offers convenience of low profile ofinstallation, however, the angular motion control is very complicated.The first rotation axis is typically perpendicular to the localhorizontal plane, a platform is built rotating around this axis, whichsets the azimuth of the solar tracker; and the second rotation axis isbuilt on this platform and determines the elevation of the solartracker. Rotation around both axes are nonlinear motions, usually servomotors with input from sun position sensor are used to drive thetracker.

2-axis tracking can also be done in polar coordinate system, or calledequatorial mount. It offers convenience of simple tracking. The firstrotation axis is typically parallel to the rotation axis of the Earth, aplatform is built rotating around this axis; and the second rotationaxis is built on this platform and follows the seasonal change of thedeclination of the Sun. Rotation of the first axis is very close toconstant speed of one turn per day, usually a clock motor is good enoughto drive this axis; rotation of the second axis is sinusoidal and veryslow, oscillates once per year, occasional manual adjustment or somekind of automatic rotation were proposed.

In U.S. Pat. No. 4,202,321, Volna described a hybrid solar trackingdevice, which he used both local and polar coordinate systems. He usedazimuth (axis 13) and elevation (axis 16) angles in a local coordinatesystem to define the orientation of his solar energy collector 29,however, he used polar axis 23 and declination “point axis 27” todescribe his drive mechanism in a polar coordinate system, and usedspindle 26 to connect both as “A coordinate transformation apparatus”(claim 8). In this way, he avoided the complex rotation control in thelocal coordinate system. All four axes: azimuth axis 13, elevation axis16, polar axis 23, and declination “point axis 27” need to intersect atthe same point so that he could use it as an axis converter. In thepolar coordinate system, he proposed a generic concept of manual orautomatic adjustment of the declination angle by sliding the “spindle26”, hence the “pointing axis 27”, back and forth by ±23½°. In column 4,line 49, “Alternatively, in a more sophisticated and automaticembodiment of the invention, appropriate drive mechanisms may beconnected to automatically slide bearing blocking 25 over datum surface24 in timed relationship with the days of the year.” However, he did notgive any specific method to implement such automatic adjustment of thedeclination angle. Also, Volna's implementation is an axis converter,not a polar tracking device, it has mechanical limitation of rotationsthat the device he proposed could not rotate over 360°, or rotateperpetually since the circular sector arm 20 collides with the pedestal11 at certain angle. It has to rotate back and forth each day, whichdefeats one of the major advantages of polar tracking.

In U.S. Pat. No. 4,402,582, Rhodes described a polar tracking device anda method to automatically adjust the declination angle. However, thecontinuous declination angle adjustment mechanism is only approximatelysinusoidal, there is no way mentioned to correct such error. Inaddition, the gear ratio is set to be 365:1, which will lead toaccumulation of error since a year is not exact 365 days, and it is noteasy to implement a more accurate gear ratio into such parasitic drivenapparatus. Also, the parasitically powered adjustment only works when itrotates continuously, it cannot have the option of rotating back andforth.

In U.S. Pat. No. 4,368,962, Hultberg described a polar tracking deviceand a more sophisticate method to adjust the declination angle, and hefurther implemented error correction mechanism to correct various errorswhich compensates the errors from the imperfect drive train,eccentricity of the earth's orbit around the sun, etc. It is a verycomplicate implementation with too many gears, and multi-section coaxialrotations. The mechanical link “space crank” requires that four axes tointersect at one point. All make it difficult to practice.

3. Objects and Advantages

To overcome the limitation of solar tracker in these prior arts, and tosimplify the implementation method, first, a 1-axis solar tracker inpolar orientation with a latched declination angle is invented, and thisdeclination angle is only updated abruptly periodically andautomatically, it offers the simplicity of implementation and reasonablygood tracking accuracy. Second, a solar tracker with slight differentimplementation that incorporates differential coaxial rotation isinvented, it is particularly applicable to polar tracking, and could befurther generalized to any 2-axis solar tracking configuration.

SUMMARY

As known in the prior arts, polar tracking has advantage of simpledriving and control requirement, while the rotation around the polaraxis is almost at steady speed: 360 degrees per day, the declinationangle change is very slow, only changes ±23½° back and forth per year.It would be a good approximation that the declination angle could bekept constant for a day. As a matter of fact, the declination anglechange rate varies, its fastest change rate is less than ±0.1° for ±6hour, while the sun itself has a ±0.267° angular size. So if we couldset the declination angle at correct value, keep it fixed through theday while we track the sun using only one axis polar tracking, we stillhave good enough tracking accuracy. Of course we have to set the nextcorrect declination angle for the next day, a manual adjustment istedious, a continuous adjustment is an overkill as others alreadyproposed in the prior arts and is unnecessary. An automatic,non-continuous, and abrupt adjustment of the declination angle of thepolar tracker is an object of the present invention.

The proposed apparatus has a rotatable shaft orientated substantiallyparallel to the rotation axis of the earth. One or more crossbars arerigidly and perpendicularly attached to this shaft, solar energycollectors are mounted on these crossbars and could rotate around thesecrossbars to define different declination angle. Those solar energycollectors are connected to a set of gears by a mechanical link, anddifferent position of the gears determines the declination angle of thesolar energy collectors. One gear in the gear chain is usually latched,for example, by a spring load ball against a notch, which causes thewhole gear chain and the declination angle to be kept at a fixedposition. When correctly forced at the correct time, the latched gearwill be unlatched, turned to the next correct position, and reapply thelatch. In this example, the spring will be compressed, the ball yieldsits way of blocking the notch so that the gear could rotate until theball falls into the next notch; at that time, the external force isremoved and the spring loaded ball latches the gear at the new position,which defines the next declination angle for the solar tracker.

This 1-axis polar tracking with automatic and non-continuous declinationangle update should provide enough accurate sun position tracking withsimple implementation requirement. In order to further improve sunposition tracking, this non-continuous declination angle adjustment isnot enough. This basic tracking apparatus can be modified to implementmore error correction mechanism, a coaxial differential rotation methodcould achieve that goal, with both rotations near constant speeds. Whilethe main shaft with solar energy collector(s) rotates at the constantspeed of one turn a day, the same as mentioned above, a gear thatrotates coaxially either faster or slower than the main shaft, thisrelative motion, goes though mechanical link, can continuously turn thedeclination angle. Both rotation speeds and durations can be controlledby simple counters, that small adjustments of rotation speeds and/ordurations could easily provide error correction for earth's eccentricorbit around the sun and for all other causes, both yearly and daily.

DRAWINGS Figures

FIG. 1 is a perspective view of a 1-axis solar tracker,

FIG. 2A is a zoom in view of the gear with self-latch mechanism,

FIG. 2B shows a cross-section view of the self-latch gear, when it is inlatched position,

FIG. 2C shows a cross-section view of the self-latch gear, when thelatch starts to yield under force,

FIG. 2D shows a cross-section view of the self-latch gear, when it is inun-latched position,

FIG. 2E shows a cross-section view of the self-latch gear, when it islatched in a new position,

FIG. 3A shows a perspective view of the worm gear starts to engage withthe “open worm tooth”,

FIG. 3B shows a bottom view at start of the engagement of the worm gearwith the open worm tooth,

FIG. 3C shows a bottom view at the finish of the engagement of the wormgear with the open worm tooth,

FIG. 4 is a perspective view of a 2-axis solar tracker,

REFERENCE NUMERALS

-   -   10 rotatable shaft,    -   11 driving motor,    -   12 lower support,    -   14 upper support,    -   15 polar axis,    -   20, 22 crossbars,    -   30 solar energy collector,    -   35 plate,    -   37 universal joint,    -   40 rod,    -   50, 70 worm gears,    -   51, 65 gears,    -   52, 62 beams,    -   60 worm,    -   61 motor,    -   63 slot,    -   64 spring,    -   66 ball,    -   71, 72, 73 inner teeth of a worm gear,    -   80 open worm tooth,

DETAILED DESCRIPTION

A 1-axis polar solar tracker is shown in FIG. 1, a rotatable shaft 10 isinstalled between a lower mount 12 and an upper support 14. Both 12 and14 are fixed on the ground, and are installed in such a way that theshaft 10 can rotate along axis 15, which is essentially parallel to thecelestial rotation axis of the Earth. Crossbars 20 and 22 are rigidlyattached to the rotatable shaft 10, and the crossbars 20 and 22 areperpendicular to the rotatable shaft 10. There are more support beams 52and 62 rigidly attached to the rotatable shaft 10. Solar energycollector 30 is mounted on the crossbar 20 and it could rotate aroundthe crossbar 20 for at least ±23½°, which defines the declination angle.If we establish xyz coordinates here, center line of the shaft 10 asy-axis, center line of the crossbar 20 as x-axis, then we clearly knowthe z-axis is perpendicular to both x and y. the angle between thenormal of the solar energy collector surface and the z-axis is thedeclination angle. More solar energy collectors could be similarlymounted on crossbar 22 and are omitted for illustration simplicity. Oneside of the solar energy collector 30 is attached to plate 35, which ispivot connected to one end of a rod 40 by a universal joint 37, theother end of the rod 40 is pivot connected to a worm gear 50, which ismounted on the beam 52. As the worm gear 50 turns, when the rod 40 goesdown to the lowest point corresponds to 23.5° declination angle of thesolar energy collector 30, and when the rod 40 goes up to the highestpoint corresponds to −23.5° declination angle of the solar energycollector 30. In a very good approximation, a steady rotation of theworm gear 50 translates into a sinusoidal oscillation of the declinationangle of the solar energy collector 30 through the mechanic link 40. Theworm gear 50 is driven by a worm 60, which is connected to another wormgear 70. The worm gear 70 is mounted on beam 62, and is usually latchedto the beam 62 as explained in the next paragraph. Everything mentionedabove forms a temporary rigid body on shaft 10, and is driven by motor11 to rotate at a constant speed along polar axis 15 perpetually. Thereis an “open worm tooth” 80 which is fixedly mounted on the ground, andis shown not touch any other part.

FIGS. 2A, 2B, 2C, 2D, and 2E illustrate a generic self-latch mechanismof the gear 70 on the beam 62. FIG. 2A is a zoom-in 3D view of thetracker near the gear 70, a latching ball 66 is visible. FIG. 2B shows across-section view of the gear 70 on the beam 62. The gear 70 has thesame number of outer teeth as that of inner teeth. 20 teeth on the gear70 are shown for illustration purpose only, exact number of teeth variesby design. A spring 64 is housed inside a slot 63 which is positioned inradial direction of the beam 62, this spring 64 pushes a ball 66outward, against the notch between two adjacent inner teeth 71 and 72 ofthe gear 70, prevents the gear 70 from rotating around the beam 62freely. The strength of the spring 64 and the inner tooth slopedetermine the workload of this latch. When enough tangential force isapplied to gear 70, the inner tooth 72 will push the ball back to theslot 63, as shown in FIG. 2C. If the gear 70 continue to rotate, asshown in FIG. 2D, the spring loaded ball 66 has no latch function to thegear 70 until one tooth interval has been rotated, as shown in FIG. 2E,at this time, the ball 66 will be pushed out by the spring 64 again,positioned between the inner teeth 72 and 73 of the gear 70. If thetangential force is removed, the gear 70 is now latched at a newposition.

As seen in FIG. 1, this latched gear 70 is connected to worm 60, whichin turn drives the gear 50. The position of gear 50 determines thedeclination angle of the solar energy collector 30 through the link rod40, so the declination angle is latched to a particular value. The wholeassembly is rotating together on the shaft 10 along the polar axis 15,one turn per day drive by a single motor 11, as a 1-axis solar tracker.However, at a pre-determined time of the day, the worm gear 70 startedto be in contact with the “open worm tooth” 80 which is fixed on theground. A zoom-in 3D perspective view is shown in FIG. 3A.

The “open worm tooth” 80 is a spiral shaped tooth, its cross sectionmatches that of the worm gear 70 as seen in FIG. 3A. This “open wormtooth” 80 sits roughly in a plane that is perpendicular to the polaraxis 15. In this plane, as shown in FIGS. 3B and 3C, the distancebetween one end of the “open worm tooth” 80 to the center of the shaft10 is different from the distance between the other end of the “openworm tooth” 80 to the center of the shaft 10, the difference is onetooth pitch of worm gear 70. When the 1-axis solar tracker rotates, theworm gear 70 engages with the “open worm tooth” 80, which forces wormgear 70 to turn one notch during the engagement. Once the worm gear 70leaves the “open worm tooth” 80, the worm gear 70 is latched at a newposition, one tooth advance from the previous position.

We could choose the appropriate gear ratios so that a closeapproximation of 365.242199 turns of the shaft 10 will result in oneturn of the gear 50. In theory one stage of worm gear is enough. A2-stage worm gear is illustrated in FIG. 1 to provide a stronger latchof the declination angle enhanced by worm 60 to worm gear 50, and thislatch strength enhancement is unidirectional which is a bonus feature;and to provide more accuracy and design flexibility of gear ratiochoices. More stages of gears could be used for the same purpose; if aratchet is incorporated, the shaft 10 could rotate back and forth whilethe declination angle is still updated as if the shaft 10 rotatesperpetually. This is a one axis solar tracker in polar mount with itsdeclination angle fixed during each day, and abruptly updates itsdeclination angle daily and automatically.

To further enhance the tracking accuracy, a two-axis tracking method isneeded, that means the tracker has to adjust its declination anglecontinuously. As seen in FIG. 4, a rotatable shaft 10 is installedbetween a lower mount 12 and an upper support 14. Both 12 and 14 arefixed on the ground, and are installed in such a way that the shaft 10can rotate along axis 15, which is essentially parallel to the celestialrotation axis of the Earth. Crossbars 20 and 22 are rigidly attached tothe rotatable shaft 10, and the crossbars 20 and 22 are perpendicular tothe rotatable shaft 10. There is a support beam 52 rigidly attached tothe crossbar 10. Solar energy collector 30 is mounted on the crossbar 20and it could rotate around the crossbar 20 for at least ±23½°, whichdefines the declination angle. If we establish xyz coordinates here,center line of the shaft 10 as y-axis, center line of the crossbar 20 asx-axis, then we clearly know the z-axis is perpendicular to both x andy. the angle between the normal of the solar energy collector surfaceand the z-axis is the declination angle. More solar energy collectorscould be similarly mounted on crossbar 22 and are omitted forillustration simplicity. One side of the solar energy collector 30 isattached to a plate 35, which is pivot connected to one end of a rod 40by a universal joint 37, the other end of the rod 40 is pivot connectedto a gear 51, which is mounted on the beam 52. As the gear 51 turns,when the rod 40 goes down to the lowest point corresponds to 23.5°declination angle of the solar energy collector 30, and when the rod 40goes up to the highest point corresponds to −23.5° declination angle ofthe solar energy collector 30. The gear 51 is driven by another gear 65,which is rotating coaxially with the shaft 10. Motor 11 drive the shaft10 at a speed of one turn a day; motor 61 drives the gear 65 at adifferent speed. The differential rotation speed between the shaft 10and the gear 65, combines with the gear ratio of gears 65 and 51, turnsthe gear 51 to rotate one turn per year. For example, if the gear 51 has20 teeth and the gear 65 has 50 teeth, then the gear 65 turns at speedof (365.242199±1)/365.242199*20/50=1.00109516 or 0.9989048 turns perday.

With the help of two independent rotations of the shaft 10 and the gear65, error correction for various causes could be done. The earth orbitaround the Sun is not a perfect circle, so that the declination anglechange is not a strict sinusoidal function of time, neither is every dayexact 24 hours. The simple mechanical link 40 between the gear 51 andthe solar energy collector 30 (via the plate 35 and the universal joint37) will not translate the circular motion of the gear 51 into an exactsinusoidal angular motion of the declination angle. All small angularerrors for polar axis rotation and for declination angle change, fromabove mentioned causes and other causes not yet elaborated, can beeasily corrected by small adjustment of rotation speed and duration ofthe shaft 10 and the gear 65, which are in turn driven by motors 11 and61. A preferred embodiment is to let both motors 11 and 61 operated atconstant speeds at most time of a day, say 23 hours, speed up or down alittle bit at the remaining 1 hour to compensate any angular errors forthat particular day. Those small errors are well known and could betabulate into control programs.

If this 2-axis solar tracker is mounted in such a way that the shaft 10is not parallel to the celestial rotation axis of the Earth, polartracking assumption is no longer valid. However, with the help of twoindependent rotations of the shaft 10 and the gear 65, accurate sunposition tracking still can be achieved when it is operated as a generic2-axis solar tracker. This is particularly useful when the orientationof the shaft 10 only slightly deviates from the polar axis. In thiscase, the shaft 10 should rotate close to constant speed, and the“declination angle” of the tracker should change a small amount duringthe day. All these small variance are well known and can be tabulateinto control programs. This optional operation mode may have newapplications for this 2-axis tracker.

Operation

For the 1-axis solar tracker that is shown in FIG. 1, once the initialdeclination angle and polar angle are set correctly, motor 11 starts todrive the shaft 11 at constant speed at one turn per day, in theopposite direction of the Earth's rotation. If the gear ratio is setclose enough to 365.242199, perpetual rotation of shaft 10 will providevery good solar tracking. In practice, if the gear ratio is set to 365,then only one manual declination angle adjustment every four years isneeded, which is simply to manually turn the self latched gear 70 by onenotch once every 4 years; similarly, if the gear ratio is set to 366 or364, then 3 or 5 manual adjustments every four years are needed, so onand so forth. To correct the error due to non-uniform day length, thatis because each day is not exact 24 hour, this driving motor 11 may beprogrammed to rotate a little faster or slower from day to day tocounter such variance, or only to rotate a little faster or slowerduring a portion of night time is enough to compensate the day lengthvariance problem.

For the 2-axis solar tracker that is shown in FIG. 4, once the initialdeclination angle and polar angle are set correctly, motor 11 starts todrive the shaft 10 at constant speed at one turn per day, and motor 61starts to drive the gear 65 at constant speed slightly faster or slowerthan one turn per day. In the previously mentioned example, with gearratio of 50 to 20, gear 65 rotates at constant speed of 1.00109516 or0.9989048 turns per day. Perpetual rotation of shaft 10 and gear 65 willprovide very good solar tracking; rotate back and forth will also do thejob. To correct the error due to non-uniform day length, that is becauseeach day is not exact 24 hour, the driving motor 11 may be programmed torotate a little faster or slower from day to day to counter suchvariance, or rotates a little faster or slower during a portion of nighttime is enough to compensate the day length problem. To correctnon-exact sinusoidal declination angle change, the driving motor 61 maybe programmed to rotate a little faster or slower from day to day tocounter such variance. The error correction program can be furtherextended to counter the non-uniform rotations when the shaft 10 ismounted not exactly parallel to the celestial rotation axis of theEarth, the variance is well known.

1. A 1-axis solar tracking device consists: a rotating shaft with one ormultiple crossbars perpendicularly attached to it, this shaft is mountedalong the celestial rotation axis of the Earth, rotates continuously andperpetually at constant speed of one turn per day, in the oppositedirection of the Earth's rotation solar energy collectors mounted on thecrossbar and can rotate around this crossbar for at least ±23½°, thisrotation defines declination angle, a self-latch mechanism keeps thedeclination angle constant most of the day, an automatic andnon-continuous declination angle update mechanism.
 2. The 1-axis solartracking device of claim 1, the mechanism that determines thedeclination angle of the solar energy collectors consists: a gear, whichis mounted on a beam that is rigidly attached to the main rotatingshaft, a rod with one end pivot connected to this gear and the other endpivot connected the solar energy collector, a steady rotation of thegear translates into an approximate sinusoidal oscillation of thedeclination angle of the solar energy collector through the mechaniclinking rod, the amplitude of the oscillation of the declination angleis 23½°.
 3. The 1-axis solar tracking device of claim 1, the self-latchmechanism that keeps the declination angle constant consists: a springloaded ball pushes against the notch between two adjacent teeth of agear, prevents the gear from rotating freely, the strength of the springand the gear tooth slope determine the workload of this latch, anadditional worm gear stage provides a unidirectional and stronger latchof the declination angle,
 4. The 1-axis solar tracking device of claim1, the automatic and non-continuous declination angle update mechanismconsists: an open worm tooth which is spiral shaped, its cross sectionmatches that of the worm gear, the open worm tooth which is fixedlymounted on the ground, sits roughly in a plane that is perpendicular tothe main rotating shaft, the distance between one end of the open wormtooth to the center of the main rotating shaft is different from thedistance between the other end of the open worm tooth to the center ofthe main rotating shaft, the difference is one tooth pitch of the wormgear, during most time of a day, the open worm tooth does not engagewith anything; at a pre-determined time of the day, the open worm toothengages with the worm gear, and forces the worm gear to turn one notchduring the engagement.
 5. The 1-axis solar tracking device of claim 1,once the initial declination angle is set correctly, and the gear ratiois set close enough to 365.242199, perpetual rotation of the mainrotating shaft will provide a very good solar tracking. However, if thegear ratio is set slightly different from the ideal number 365.242199,periodical manual adjustments is needed to reset the accumulated errorin declination angle. For example, if the gear ratio is set to 365, thenonly one manual declination angle adjustment every four years is needed,which is simply to manually turn the self latched gear by one notch onceevery four years; similarly, if the gear ratio is set to 366 or 364,then 3 or 5 manual adjustments every four years are needed, so on and soforth.
 6. The 1-axis solar tracking device of claim 1, to correct theerror due to non-uniform day length, that is because each day is notexact 24 hour, the main rotating shaft may be programmed to rotate alittle faster or slower from day to day to counter such variance, oronly to rotate a little faster or slower during a portion of night timeis enough to compensate the day length variance problem.
 7. A varianceof the 1-axis solar tracking device of claim 1, if a ratchet isincorporated in the declination angle adjustment mechanism, the mainrotating shaft could have an option to rotate back and forth while thedeclination angle is still updated as if the main rotating shaft wasrotate perpetually, with all the benefit of its declination angle fixedduring each day, and abruptly updates its declination angle daily andautomatically.
 8. A 2-axis solar tracking device consists: a rotatingshaft with one or multiple crossbars perpendicular attached to it, thisshaft is mounted essentially along the celestial rotation axis of theEarth, rotates continuously and perpetually at constant speed of oneturn per day, in the opposite direction of the Earth's rotation, solarenergy collectors mounted on the crossbar and can rotate around thiscross bar for at least ±23½°, this rotation defines declination angle, aco-axial rotation mechanism, though mechanical link, changes thedeclination angle continuously, an error correction program controlsboth rotations.
 9. The 2-axis solar tracking device of claim 8, themechanism that determines the declination angle of the solar energycollectors consists: a gear, which is mounted on a beam that is rigidlyattached to the main rotating shaft, a rod with one end pivot connectedto this gear and the other end pivot connected the solar energycollector, a steady rotation of the gear translates into an approximatesinusoidal oscillation of the declination angle of the solar energycollector through the mechanic linking rod, the amplitude of theoscillation of the declination angle is 23½°.
 10. The 2-axis solartracking device of claim 8, the mechanism that continuously changes thedeclination angle consists: a gear, which rotates coaxially with themain rotating shaft, engages with the other gear whose rotation causesthe oscillation of the declination angle, two independent drivingmechanisms that drive this gear and the main rotating shaft, thedifferential rotation speed between this gear and the main rotatingshaft, combines with the gear ratio in the drive train, continuouslychanges the declination angle.
 11. The 2-axis solar tracking device ofclaim 8, the error correction program controls both rotations canslightly adjust both rotation speed and/or duration; that willcompensate polar tracking errors from all sources. Those small errorsare well known and could be tabulate into control programs.
 12. Avariance of the 2-axis solar tracking device of claim 8, the drivingmechanism have an option to rotate the main rotating shaft back andforth with all the benefit of accurate polar tracking during thedaylight time.
 13. A variance of the 2-axis solar tracking device ofclaim 8, the error correction program can be extended to countertracking error when this 2-axis solar tracker is not mounted in polarorientation.